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Full text of "Laboratory spray chamber for producing deposits of limited drop size"

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March 1951 ET-293 

United States Department of Agriculture 

Agricultural Research Administration 

Bureau of Entomology and Plant Quarantine 



C. B. Eaton and J. P. Secrest 
Division of Forest Insect Investigations 

During the course of investigations of aerial spray deposits at 
Beltsville, Md.,— it was found desirable to reproduce, under controlled 
conditions in the laboratory, deposits similar to those obtained in aerial 
spraying. The common boom-and-nozzle type of spray apparatus used 
on airplanes for forest-pest control produces drops with a mass median 
diameter of about 300 microns. An apparatus was constructed to deliver 
a spray deposit comprised of drops this size or smaller. 


The settling-tower type of spray chamber such as is commonly used 
for testing insecticides (Campbell etal. J_ and Waters 3^) was not suitable 
for these tests. In it the spray is introduced at the top and settles to the 
bottom with drops of all sizes intermingled. In these investigations the 
primary objective was separation of the large drops from the small ones 
in concentrated solutions such as are used in forest spraying. This was 
accomplished by constructing a chamber in which the spray can be intro- 
duced at one end. As the spray is introduced uhder pressure, the larger 
drops are driven farther than the smaller ones; thus, as they settle, 
there is a gradation in drop size from end to end of the chamber. Samples 
of deposit taken at different distances from the point of introduction of the 
spray give different drop spectra. 

This chamber is constructed mainly of 24-gage aluminum on a wooden 
base. Its dimensions are shown in figure 1. The base inside the chamber 
is crowned slightly to insure drainage of sprays and has two square holes 
beneath which deposits may be collected. A nozzle and a shutter are located 

J_/ In cooperation with the Division of Farm Machinery, Bureau of 
Plant Industry, Soils, and Agricultural Engineering. 

at one end of the chamber (fig. 2). They are enclosed to confine the 
excess spray (fig. 3), and provision is made for catching the run-off 
from the shutter. 

The nozzle used is a modified nose-and-throat atomizer. It is pre- 
pared by substituting a metal tip and a straight 4-inch metal fluid -intake 
tube for the original plastic ones and by inserting a No. 22 surgical 
intramuscular needle into the air outlet to constrict that opening. The 
end of the air tube is aligned with the outlet hole in the outer tube in order 
to insure a uniform spray pattern. The fluid-intake tube of the atomizer 
is inserted in a reservoir of spray material, the level of which is kept 
constant from an inverted flask above. The arrangement of nozzle, 
shutter, and reservoirs is shown in figure 2. Air for the atomizer is 
provided by a small compressor with a ballast tank and reduction valve 
from a portable air tank or from a cylinder of carbon dioxide. 

A pendulum type of shutter controls admission of the spray stream 
into the chamber. It is attached to a solenoid, which opens it against a 
spring tension. It is opened and closed through a system of relays and 
mechanical contacts accentuated by flexible rubber tips projecting from 
a phonograph turntable operating at 78 revolutions per minute (fig. ^).— 
The distance between the points on the turntable is variable, so that the 
shutter can be held open for different periods of time. The shutter can 
also be operated by hand. 


The spray chamber is calibrated for drop size and for deposit volume. 
In calibration for drop size deposits are collected on microscope slides 
that have been polished after being coated with a silicone preparation. 
The lens diameters of the drops are measured under a microscope by 
means of an optical micrometer. Drop diameters are calculated from 
the lens measurement by use of the following formula: 

Lens diameter (microns) = drop diameter (microns) 
Spread factor 

The spread factor is a constant for any one spray material and is 
determined from measurements of a drop formed on a polished surface^ 
the focal length of the lens, and the index of refraction of the material. 
A graph for determining it is given by Davis {2, fig. 5), 

The volume of the deposit is controlled by varying the shutter timing, 
and any convenient means of analyzing the deposit may be used. 

2/ The authors are indebted to J. M. Davis for the design and con- 
struction of the timer for the spray chamber. 


This apparatus, when used with an air pressure of 20 pounds per 
square inch, delivers drops averaging 300 microns in diameter at the 
opening farthest from the nozzle and 80 microns at the nearest opening. 
Drop size can be varied by changing the distance between the end of the 
air tube and the spray outlet hole at the nozzle tip. Some atomizers can 
be made adjustable at this point. Drop size can also be varied to a limited 
extent by changing the pressure. Increasing it tends to reduce the drop 
size. Either of these adjustments for drop size will cause slight changes 
in nozzle output, as will any change in the fluid level or head in the lower 
reservoir during operation. 

Use of the atomizer type of nozzle limits the apparatus to solutions 
and emulsions. Stable emulsions have been used without any agitation. 
Suspensions have not been used satisfactorily with the atomizer. However, 
hydraulic nozzles that will accommodate suspensions can be used. 

The time the shutter is held open, and consequently the volume of 
spray deposited, depends upon the distance between the actuating tips on 
the turntable. This distance is easily changed by moving one of the 
suction cups on the glass top of the turntable. Spray deposits are made 
on glass plates and analyzed after each change in timing until the desired 
deposit is obtained. For use with oil solutions of DDT a dye tracer method 
with Dupont oil red has been found most convenient. A direct-reading 
colorimeter of the Duboscq type makes possible rapid checks on the 
deposit throughout a series of tests. With drops smaller than 100 microns 
a draft shield around the falling spray is necessary to prevent drifting. 

The spray chamber should be operated where there is plenty of 
ventilation. With continuous operation the finer portions of spray, which 
are of aerosol size, may escape into the room as a source of contamination 
of materials and equipment and discomfort to the operator. 

This apparatus has proved very effective in the production of spray 
deposits within the range of those commonly produced by aircraft apparatus. 
It has been used in screening various spray formulations with relation to 
mortality and dosage and in drop-size studies. With it deposits have been 
placed on all sorts of surfaces including that of foliage. It has been of 
limited value in treatment of foliage, however, because of the small areas 
that can be covered effectively. 

Literature Cited 

(1) Campbell, F. L. , Earnhart, C. S., and Hutzel, J. M. 

1941. Tests on crawling insects. Soap 17(7): 109-111. 

(2) Davis, J. M. 

1949. A photographic method for recording size of spray drops. 
U. S. Bur. Ent. and Plant Quar. ET-272, 7 pp. 

(3) Waters, Harold A. 

1943. General methods and equipment. In Campbell, F. L., and 
Moulton, F. R., eds.. Laboratory procedures in studies 
of the chemical control of insects. Amer. Assoc. Adv. 
Sci. Pub. 20, pp. 115-123. 



^— 6"-^ 

k— 6"-J 










|« 10" 

Figure 1. --Diaf^ram of spray chamber, showina dimensions. 


Figure 2. --End view of spray 
chamber, showing arrange- 
ment of shutter, nozzle, and 
spray reservoirs. 

Figure 3. --Spray chamber, including housing for parts shown in figure 2, 
and with draft shield in place. 

Digitized by the Internet Archive 
in 2013 

78 RPM. 












#1. Auto-horn relay. 

#2. Same, altered for separate 

circuit across contacts. 
#3. Sisson choke. 


Figure 4. --Timer and circuit diagram. 



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